1. Field of the Invention
The present invention relates to an improvement in an ultrasonic transducer using a laminated piezo-electric element and more particularly to an ultrasonic transducer with improved directivity characteristics and improved sensitivity without losing transient characteristics (pulse characteristics) and is suitable, for example, for supersonic distance measurement.
2. Description of the Prior Art
Ultrasonic transducer for use in the air has been proposed and includes laminated piezo-electric ceramic elements which are designed to work at resonance point or anti-resonance point. Further, since the mechanical impedance of air is very much smaller than that of the peizoelectric ceramic element, the laminated element is connected to a diaphragm for attaining mechanical impedance matching therebetween.
For instance, in a video camera having an automatic focussing mechanism for its objective lens by means of ultrasonic distance measurement, the measurement must be made continuously. Such continuous measurement requires a good transient characteristic in order to avoid error in measurement. For such good transient measurement, short rise and fall times are necessary. On the other hand, in such video camera using zoom lens as an objective lens, a distance measurement for such zoom lens must be made with a sharp directivity corresponding to the narrowest picture angle of the zoom lens.
Hitherto, a ceramic ultrasonic transducer has been known as the apparatus of a high sensitivity, high durability against moisture or acidic or salty atmosphere and high S/N ratio due to its resonance characteristic. But the ceramic ultrasonic transducer has had bad transient characteristic due to its very high mechanical Q value.
A typical example of conventional ultrasonic transducer is shown in FIG. 1, which is a sectional elevation view along its axis. As shown in FIG. 1, a lower end of a coupling shaft 2 is fixed passing through a central portion of a laminated piezo-electric element 1 with the upper part secured to a diaphragm 3. The laminated piezo-electric element 1 such as a ceramic piezo-electric element is mounted at positions of nodes of oscillation via a flexible adhesive 5 on tips of supports 4. Lead wires 9, 9' of the laminated piezo-electric element is connected to terminals 6, 6'0 secured to base 71 of a housing 7, which has a protection mesh 8 at the opening thereof. And an outer casing 10' is formed integral with a horn 10.
FIG. 2 is a directivity diagram showing directivity for ultrasonic wave of the transducer of FIG. 1, wherein driving frequency is 40 KHz and the diameter of the horn opening is 42 mm.
In the example of FIG. 1, the half width angle and intensity of a first side lobe are calculated as 16.4.degree. and -17.6 dB, respectively, but in an actual transducer it is difficult to realize a value smaller than these values. If a high resolution for an object is intended to be achieved, a sharp directivity characteristic is required. A sharp directivity characteristics is obtained as is well known by increasing sizes of sound source i.e. diaphragm size or by raising frequency to be transmitted. However, if the frequency to be transmitted is raised, attenuation of the ultrasonic wave becomes larger. Then, when a laminated piezo-electric element is used, the ultrasonic transducer loses its sensitivity, and therefore the raising of the frequency should be limited. And in actual case, the size (i.e. the diameter) of the ultrasonic source must be made larger. Besides, when the laminated piezo-electric ceramic is used and very sharp directivity characteristics are required, the diaphragm, the laminated piezo-electric element and the base to support the piezo-electric element become very large. On the other hand, when a large diaphragm is used in order to realize a sharp directivity characteristic and thereby a high sensitivity, it is difficult to obtain an ideal piston vibration of the diaphragm, and accordingly the sensitivity or directivity characteristic is not improved much. In order to obtain a sharp directivity characteristic, there is another way of adding a horn before the diaphragm. But when a large diaphragm is used for a high sensitivity of transmission and receiving, a sharp directivity is hardly obtainable even by use of such horn.